The present invention relates to an improvement in blow tube units which are used in blow plate assemblies. A blow plate assembly is described generally in U.S. Pat. No. 3,461,948 to Londal et al. The blow tube unit described in Londal et al. is used in conjunction with pressure applied during a blowing cycle to communicate a thermosetting fluent material such as sand from a reservoir to an associated sand core mold. The sand that is being conveyed has a moisture content. As a result, at the end of the blow cycle when the pressure is released, the sand typically compacts within the blow tube unit and stops flowing into the mold.
When a fluent thermosetting material such as dry coated shell sand is used however, no compaction takes place since this type of sand has very little if any moisture content. Therefore, the shell sand continues to communicate between the reservoir and core mold via the blow tube unit even when no blow cycle pressure is applied. This is undesirable, particularly since only a fixed amount of sand is needed to form a sand mold.